Most skeletal muscles consist of variable mixtures of structurally different "fiber types", but the significance of this heterogeneity is not well understood. By using immunocytochemical procedures to localize the contractile proteins with respect to individual muscle fibers, we are attempting to determine the relationship between the pattern of distribution of the fibers and the chemical composition and physiological properties of the muscles. We have shown, in the rat, that fibers can be distinguished on the basis of immunochemical differences in their myosins and also the proteolytic subfragments and light chains of myosin. We have recently begun to analyze the distribution of myosin isoenzymes in selected skeletal muscles of the cat, since physiological data have already been obtained in this animal. Our observations using innumocytochemistry suggest that there is a correlation between the type of myosin present in a particular fiber and its speed of contraction. We are slso examining, by the combined approaches of immunocytochemistry and two-dimensional gel electrophoresis, the changes in distribution of fast and slow myosins that occur during normal development and following denervation. These studies should provide basic information on the contractile machinery of individual skeletal muscle fibers, which is essential to an understanding of the physiological and pathological alterations that occur in skeletal muscles.